Font's in Gerben Wierda's TeX distribution and ConTeXt

["Pepe Barbe" <elventear@gmail.com>]

Hello,

I am using GW's TeX distribution with ConTeXt. I am interested in
using a different font other than Computer Modern. This distribution
seems to come with a ton of fonts (More than I need at least). Using
the basic ConTeXt provisions to access the fonts I am able to access
some fonts successfully (Like Palatino or Euler) and some
unsuccsessfully (Like Optima, Iwona or Kurier).

Ex:
Works:
\definetypeface [MyFace] [rm] [serif] [palatino] [default] [encoding=ec]

Does not work:
\definetypeface [MyFace] [ss] [sans] [iwona] [default] [encoding=ec]


I realize this might be a very basic question but: What's going on? :-)

I would appreciate if anyone could at least point me in the direction
of some documentation that would explain why these fonts work/don't
work.

Thank you,
Pepe